Why this set deserves to be on-site:
Tornadus is only known for it's signature move, hurricane. Many people just throw on a specs and unskillfully destroy the opposing people. Specs Hurricane is actually able to 2HKO 48 out of the 53 OU pokemon. The remaining 5 pokemon are Tyranitar, Chansey, Blissey, Heatran, and Jirachi. This is an incredible feat that no other pokemon acheived. The reason mixed Tornadus could be better is because Tornadus can do much more damage to it regular checks, which happen to be very common. In addition, it can stop many stallers with taunt and can save your ass or just help you sweep with tailwind.

Hurricane actually does not lose much momentum. With stealth rocks, it is able to OHKO the following pokemon(assuming no incredibly unusual set such as a 252 Special Defense Scizor) in the OU metagame:
Alakazam
Cloyster
Mienshao
Mamoswine
Breeloom
Forretress
Conkeldurr
Dugtrio
Toxicroak
Haxorus
Lucario
Infernape
Gliscor
Gengar
Landorus
Salamence
Starmie
Hydreigon
Terrakion
Espeon
Venusaur
Celebi
Volcarano
Scrafty
Virizion
Remember, this is with stealth rocks. Mamoswine, Starmie, and all scarf sets could revenge kill Tornadus though. LO Lucario extremespeed does around 53%.

As you can see, with the help of SR, Tornadus actually still retains most of it's OHKOs.

The Mixed part comes in now. Being a rain support pokemon, Tornadus has to be able to take out the two biggest enemies of Rain, Sun and Sand.

The EVs for this set guarantees a kill on a 252HP 0 Def Tyranitar with Hammer Arm. The great part is that your opponent does not usually expect this and I have been able to destroy so many T-tars doing this. Surprising, almost no one puts any evs in Def as I never failed to OHKO.

Against defensive calm 252/120 Ninetails, Hurricane will do 57%-64%. If you manage to hit Hurricane despite the accuracy, a hammer arm will do 39%-46%. So if ninetails start off with less than 93% health and you hit a hurricane, you can get rid of that Ninetails with a 90% success rate with hammer arm.

Against of Tornadus's three attacks will do incredible damage to Abomasnow with Hammer arm and Hurricane OHKO. Although +252 Atk ice shard could do up to 64% and with LO or expert belt, Tornadus pretty much dead.

Against heatran, no investment heatran will suffer 36-43% from a hurricane and Hammer arm will always finish the job.

Bulky Heatran is a little more tough, but if you have Stealth rocks and maybe is it's already weakened, you can kill it with hammer arm. It will do 54-63 to 252/0 Bulky Heatran.

Fat Blobs:

Keep in mind that because Hammer Arm is super effective, to take the least damage as possible from Tornadus's mixed attacks, Blissey and Chansey will need to invest as much as possible in Def since that's where it will hit the hardest.

Against +252/252 Blissey, a Hurricane will do 26%-31% and Hammer arm will do 55-64%. At minimum, it will do 81% and at most 95%. If Blissey's health is below 76 before the hurricane, Blissey is pretty much guareenteed a death.
You can also use taunt to prevent Blissey from recovering. This will allow you to fire off two Hammer arms. Although with seismic toss and LO recoil, Tornadus will probably end up dead.

Against the same Ev Spread chansey, a Hurricane Hammer arm combo will do 63-77%.

Taunt could be useful, but it's not the best for this set.

Finally, one of the best moves that Tornadus has and the one that makes Tornadus unique is Tailwind. I have never seen anyone use Tailwind competitively and it could be due to the fact that not many pokemon could use it effectively. Then, I found Tornadus. With all the residual damage, Tornadus will be left very weary, but before Tornadus dies, it can help my next pokemon sweep with tailwind. With prankster, I can always ensure that I get a tailwind before Tornadus dies. No matter how fast my opponent is. I usually place a very powerful banded or specs late game sweeper. My favorite is the tailwind Banded Salamence combo. Late game, when all the steels and priorities are dead, a 658 speed with 550 Atk beast that grows even stronger after every attack, I can easily sweep through 3 pokemon.

Aside from helping sweep. Tailwind can also act as a last resort move. If you say let a moxie Gyarados set up two Dragon Dance, you can sacrifice Tornadus and Revenge kill the Gyarados so that you can still have a chance.

Teammates & Counters:
With u-turn, you can easily trap a Heatran or Jirachi with Dugtrio. With those two pokemon gone, Tornadus can pretty much attack unresisted in OU.

For tailwind, use a strong late game attacker.

Remember, mixed Tornadus is not made to revenge kill. It mainly pressures the opponent by making them give up a pokemon. For example I switch to Tornadus while the opposing Celebi roosts. Now, my opponent wants to keep celebi so that sent out Gastrodon. Hurricane easily does well over half and Gastrodon is pretty much dead. The funny part is that I can usually bring out Tornadus many times against the same pokemon in one game. Imagine that. Every time Tornadus comes in, you have to sacrifice one pokemon. Again, Tornadus keep U-turn to trap its only counters, Jirachi and Heatran.

Can you imagine facing this thing. Nothing counters tornadus, but only check it. If tornadus comes in on my forretress or celebi, I would have to either switch into Heatran fearing trapped by Dugtrio or switch into Rotom-W and lose over 40%.

Revenge killing it is easily and without Rocks and LO, Tornadus should wear down. Although Tailwind is extremely hard to deal with since it can unpredictable.